The present invention relates to optical filters for special purpose lenses. More specifically, the present invention relates to a method for designing a filter with regard to how effectively it enhances the contrast between different shades of the same spectral color.
In the optics field, color measurement is an extensive subject on which many treatises and books have been written. Color measurement is a useful tool in many areas of industry, examples of which include color discrimination for quality control; contrast enhancement in CRT screens; testing of samples for non-conformities (e.g., forged instruments); and color matching of paints and other colored products.
The color of an object is based in part on how the object absorbs the visible spectrum of light. While certain wavelengths of light are absorbed, other wavelengths are reflected which impinges upon the retina of the human eye. The human eye has three photopigments located in the cones of the retina, which allow for color perception: protos for red; deuteros for green; and tritos for violet. Stimulation of the three photopigments in varying proportions and combinations provide sensitivity and perception to all other colors in the electromagnetic radiation spectrum. The wavelength range of color in the electromagnetic spectrum is widely considered to be between approximately 390 nm and 770 nm.
The wavelength bandwidth of each spectral color is regarded in the art generally as follows:
TABLE 1 ______________________________________ Spectral Color Violet Blue Green Yellow Orange Red ______________________________________ Wavelength 390- 455- 492- 577- 597- 622- 455 nm 492 nm 577 nm 597 nm 622 nm 770 nm Bandwidth ______________________________________
The human eye does not perceive all wavelengths of color evenly. Rather, the normal human eye response peaks at approximately 560 nm (green), sloping off to the right and left of the spectrum, resulting in what is referred to in the art as the bell-shaped photopic response curve of the human eye (see FIG. 1). The human eye can thus perceive the color green more effectively than other colors of the spectrum. It is also known, however, that not all people perceive colors exactly the same. For example, a common defect of eye, referred to as "color blind", causes some people to confuse the colors red and green. Color perception is also affected by the illuminating light source.
Due to differences in ambient lighting conditions and color perceptions between different people, the International Commission on Color Illumination ("CIE") was founded to help develop color matching standards for use by those working in the color matching industry. These CIE standards are used in conventional color matching equations. For example, there is the CIE standard observer 2.degree. (small (fovea) sample) and 10.degree. (large (retina) sample), as well as "standard illuminants" D.sub.65, A and C. These standards, together with direct measurement of the reflectance (R) of a sample using a spectrophotometer, are parameters used in well-known color matching calculations to provide the CIE Color Scale Parameters X, Y and Z (also referred to as "tristimulus" values). The standard observer is represented as x, y and z in the color matching equations for the three additive primaries red, green and blue, respectively. The tristimulus values X, Y and Z are used to yield color coordinates x and y for plotting on the CIE chromaticity coordinate graph which defines the color space of the sample in two dimensions. The basic color matching equations are as follows: EQU X=.SIGMA.S.sub.(.lambda.) .times.R.sub.(.lambda.) .times.x.sub.(.lambda.) EQU Y=.SIGMA.S.sub.(.lambda.) .times.R.sub.(.lambda.) .times.y.sub.(.lambda.) EQU Z=.SIGMA.S.sub.(.lambda.) .times.R.sub.(.lambda.) .times.z.sub.(.lambda.)
Where S.sub.(.lambda.), x.sub.(.lambda.), y.sub.(.lambda.) and z.sub.(.lambda.) are standardized CIE values and R.sub.(.lambda.) is the measured reflectance of the object.
And for the chromaticity coordinates: ##EQU1##
It is known that perceived color can be altered by changing the spectral response curve of the sample through the use of filters. Filters act to change the spectral response curve by altering the transmission of chosen wavelengths of color. The term "contrast enhancement" has been used by some in the art with respect to distinguishing between different spectral colors. There have thus been filters proposed which block wavelengths located between certain colors so as to eliminate the wavelengths which "bridge" two or more spectral colors of interest, thereby enhancing the contrast between the colors of interest. See, for example, U.S. Pat. No. 3,877,797 issued to Thorton, Jr. on Apr. 15, 1975. The '797 patent teaches enhancing color discrimination between differently colored objects with a filter which blocks selected bands of color wavelengths, for example at 490 nm and 590 nm.
The use of filters to alter spectral response curves has applications in the eyewear industry as illustrated by the '797 patent. Other filters used in eyewear applications may be seen in the following patents:
U.S. Pat. No. 4,802,755 issued to Bausch & Lomb Incorporated on Feb. 7, 1989 PA0 U.S. Pat. No. 5,438,024 issued to Bausch & Lomb Incorporated on Aug. 1, 1995 PA0 U.S. Pat. No. 5,190,896 issued to Schott Glass Tech., Inc. on Mar. 2, 1993 PA0 U.S. Pat. No. 5,446,007 issued to Schott Glass Tech., Inc. on Aug. 29, 1995 PA0 U.S. Pat. No. 5,077,240 issued to Schott Glass Tech., Inc. on Dec. 31, 1991 PA0 U.S. Pat. No. 5,218,386 issued to Levien on Jun. 8, 1993
It may be realized from the above patents that contrast enhancement in eyewear applications is primarily intended for "special purpose" eyewear rather than the more common "general purpose" eyewear. Special purpose eyewear is used for conditions expected at a particular environment, e.g., the bright, white conditions on ski slopes. The present assignee has marketed a special purpose lens known as "ACE" for use on the golf course which is the subject of the '240 patent above. The ACE lens is a purple lens with a high neodymium content which is known for natural absorption of spectral bands resulting in enhancement of amber, red and green. The ACE lens is thus useful for distinguishing between these different spectral colors which was thought to enhance perception of different colors found on a golf course. While this lens helped distinguish between different colors of amber, red and green, it was not specifically designed to enhance the contrast between different shades of the predominant spectral color found on a golf course, namely, green.
Another past approach to contrast enhancement in eyewear is to block the shorter wavelength blue light which is easily scattered by moisture or dust in the air on foggy or hazy days, resulting in interference with spectral colors of interest. The present assignee Bausch & Lomb marketed the B-15 Brown lens; the Ambermatic lens, the RB-50 lens, as well as other lenses in the brown/amber family to reduce haziness, improve contrast and sharpen details. Other "blue-blocking" lenses may be seen in U.S. Pat. Nos. 4,878,748; 5,177,509; and 5,400,175, all issued to Suntiger, Inc.
The above sampling of patents reveals how those skilled in the art have defined "contrast enhancement", as well as the general approach to achieving such contrast enhancement in a lens. Specifically, the term "contrast enhancement" has meant sharpening the details of a viewing field through a filter which act to reduce or eliminate chosen spectral bands to "enhance" other spectral colors of interest. While this approach has proven useful in applications where enhancement between different spectral colors is important, it has been found by the inventor herein that such an approach to filter design could actually produce deleterious effects on distinguishing between different shades of the same spectral color.
There thus remains a need for a special purpose lens which enhances different shades of the same spectral color. For example, on a golf course, there are many different shades of the color green. A golfer which "reads" the different greens correctly has an advantage over a golfer who cannot perceive subtle differences between the different shades of greens on a golf course. There are of course other environments having different predominant spectral colors for which a special purpose lens having the ability to enhance the different shades of the spectral color would be useful. Examples include sporting environments such as an outdoor sand volleyball court and ski slopes where the predominant color may be white, gray or yellow.